1. Field of the Invention
This invention relates to use of MCM-49 in catalytic cracking.
2. Description of the Related Art
Many refineries devote extraordinary amounts of energy and operating expense to convert most of a whole crude oil feed into high octane gasoline. The crude is fractionated to produce a virgin naphtha fraction which is usually reformed, and gas oil and/or vacuum gas oil fractions which are catalytically cracked to produce cracked naphtha, and light olefins. The cracked naphtha is added to the refiners gasoline blending pool, while the light olefins are converted, usually by HF or sulfuric acid alkylation, into gasoline boiling range material which is then added to the gasoline blending pool.
Fluid catalytic cracking (FCC) is the preferred refining process for converting higher boiling petroleum fractions into lower boiling products, especially gasoline. In FCC, a solid cracking catalyst promotes hydrocarbon cracking reactions. The catalyst is in a finely divided form, typically with particles of 20-100 microns, with an average of about 60-75 microns. The catalyst acts like a fluid (hence the designation FCC), and circulates in a closed cycle between a cracking zone and a separate regeneration zone. Fresh feed contacts hot catalyst from the regenerator at the base of a riser reactor. The cracked products are discharged from the riser cracking reactor to pass through a main column which produces several liquid streams and a vapor stream containing large amounts of light olefins. The vapor stream is compressed in a wet gas compressor and charged to the unsaturated gas plant for product purification.
A further description of the catalytic cracking process may be found in the monograph, "Fluid Catalytic Cracking With Zeolite Catalysts," P. B. Venuto and E. T. Habib, Marcel Dekker, New York, 1978, incorporated by reference.
An earlier process, moving bed cracking or Thermofor Catalytic Cracking (TCC), is still used in some refineries. The catalyst is in the form of small beads, which pass as a moving bed through a reactor and regenerator. The feed and product properties can be the same, but TCC units usually can crack only distilled feeds, whereas FCC can process feeds containing some residual materials.
While FCC is already an efficient process for converting heavy feed to lighter products, substantial modifications to FCC catalysts and hardware are likely to be required as a result of the 1990 Clean Air Act Amendments (CAAA). In particular, it is expected that there will be an increased demand for C3 and C4 olefins for alkylation and C4 and C5 olefins for methyltertbutyl and ethyltertbutyl ethers (MTBE and ETBE) to reduce gasoline aromatic content and increase gasoline oxygenate content. Anticipated difficulties include maintaining gasoline octane and generating enough light olefins to make oxygenates.
There are a number of widely recognized methods to increase light olefin make. For example, one widely accepted method is to substitute an ultrastable Y zeolite for a rare earth exchanged Y zeolite in the base cracking catalyst. Another is to increase the riser top temperature. A third method is to use a secondary or "quench" stream at some point along the length of the riser. Yet another method is to add ZSM-5 to the zeolite Y based cracking catalyst.
There are problems associated with each method of increasing yield of light olefins. Substituting a rare earth-free ultrastable Y zeolite for a rare earth exchanged Y produces a less stable and less active cracking catalyst. Higher riser top temperatures produce more undesirable light products such as methane and ethane and also produce more dienes in the gasoline which lead to gum formation and fouling. Introduction of a quench stream can limit the fresh feed rate on a unit close to its hydraulic limit. Addition of ZSM-5 can greatly increase light olefin yields, but adds to the cost and, if used at high concentrations, may dilute the "base" Y cracking catalyst.
Some catalytic approaches to increasing light olefin yields will now be reviewed.
Zeolite Y+ZSM-5
Use of ZSM-5 in combination with a zeolite Y based catalyst is described in U.S. Pat. Nos. 3,758,403; 3,769,202; 3,781,226; 3,894,931; 3,894,933; 3,894,934; 3,926,782; 4,100,262; 4,309,280; 4,309,279; 4,375,458 which are incorporated by reference.
Zeolite Y+Other Zeolites
Combinations of zeolite Y and other zeolites and molecular sieves including crystalline silicoaluminophosphates (SAPOs) have shown potential for increasing light olefins and octane at the expense of gasoline yield. To date, the commercial application of crystalline materials other than ZSM-5 as octane cracking catalysts appears to be limited. The scientific and patent literature includes references to the evaluation of at least four other shape selective aluminosilicate zeolites as FCC additives. These are: offretite (U.S. Pat. No. 4,992,400), ZSM-23, ZSM-35 (U.S. Pat. No. 4,016,245) and ZSM-57 (U.S. Pat. No. 5,098,555) Non-zeolitic molecular sieve patents teach the use of SAPO-5 (U.S. Pat. No. 4,791,083; EP 0 202 304 B1), SAPO-11 (U.S. Pat. No. 4,791,083) and SAPO-37 (U.S. Pat. Nos. 4,842,714; 4,681,864) in FCC.
There are references to the use of zeolite beta with zeolite Y catalysts as a means for improving gasoline octane and producing light olefins. Chen et al., in U.S. Pat. No. 4,740,292 and in U.S. Pat. No. 4,911,823, incorporated by reference, describe the use of REY+zeolite beta to improve the octane of gasoline while increasing the yield of C3/C4 olefins.
While all of the above approaches helped increase olefin yields in FCC, none provided a complete solution to the problem of making more light olefins, while maintaining gasoline yields and gasoline octane. Some required use of zeolite additives made from exotic organic templates and/or relied on use of zeolite additives which might not have the stability to survive the harsh conditions in modern FCC regenerators, which could also be correctly called hydrothermal deactivators.
We knew that cracking refineries of the future would need more olefins, and more octane. Refineries would also need to produce these fuels of the future without scrapping their existing processing units, and without major capital expense.
We discovered a new cracking catalyst, which could be used alone or in combination with conventional zeolite Y based cracking catalyst to produce extraordinary amounts of light olefins. Our new cracking catalyst, or cracking catalyst additive, dramatically increased yields of light olefins, such as propylene and butylene, and significantly increased yields of isobutane. Although there was some loss of FCC gasoline yield, this was more than offset by potential yield of liquid fuels from incremental light olefin and isobutane yields.
We achieved these benefits by using MCM-49 as a cracking catalyst additive, or as a replacement for the conventional zeolites used in the cracking catalyst.